1. Field of the Invention
This invention relates generally to porous rolls. More particularly, the invention relates to a porous roll assembly which comprises a stack of porous disks fitted on a core shaft as axially compressed. The present invention also relates to a method of making such a porous roll assembly.
2. Description of the Prior Art
Porous rolls are used for example for removing liquids from object surfaces such as the surfaces of films, steel plates, metallic foils or printed circuit boards. In use, the porous roll is held in rolling contact with the object surface, and the unwanted liquid is absorbed by the capillary action provided by minute pores of the roll. Further, when the roll is forcibly pressed against the object surface, the void volume of the roll is temporarily reduced at a portion thereof compressively contacting the object surface, so that a negative pressure is developed within that roll portion upon elastic restoration thereof following the contact. Obviously, the thus generated negative pressure greatly helps the absorptive action of the roll.
A typical porous roll comprises a core shaft and a porous roll body fitted on the core shaft. The roll body may be made of various materials such for example as non-woven fabric, porous synthetic rubber reinforced by fibers (or non-woven fabric impregnated with synthetic rubber or binder), or porous synthetic rubber alone. The core shaft may be solid or hollow.
When the core shaft is hollow, the shaft may be made to have a cylindrical wall which is formed with a plurality of radial through-holes, and one axial end (open end) of the shaft is connected to a suction device. According to this arrangement, a suction force applied to the core shaft is utilized to assist the absorptive action of the roll body, and to discharge the absorbed liquid out of the roll. Thus, the roll can be used for continuous liquid removal without requiring occasional interruption. Further, if the open end of the core shaft is connected to a liquid supply device, the roll may be also used to continuously supply a suitable liquid for intended surface treatment.
Porous rolls may be manufactured by several methods. One of such methods is described for example in Japanese patent application Laid-open No. 61-262586 (Laid-open: November 20, 1986; application Ser. No.: 60-104022; Filed: May 17, 1985; Applicant: Masuda Seisakusho Co., Ltd. and Toray Industries, Inc.; Inventors: Toyohiko HIKOTA and Masao MASUDA).
According to the method disclosed in the above laid-open application, a predetermined number of axially stacked porous disks are fitted on a core shaft, and simultaneously subjected to an axial compressive force in a single step. As a result, the pore size of the compressed porous disks is rendered far smaller than that of the uncompressed porous disks, thereby increasing the capillary ability of the roll.
The prior art method described above is acceptable as long as the length of the roll is relatively small. However, if the roll length is large, there arises a problem that the porous disks are not evenly compressed. More specifically, when axially compressing the stack of porous disks on the core shaft, the axial compressive force must be transmitted throughout the disk stack against the friction of the disks relative to the core shaft, and such a friction cumulatively increases as the axial position of the disk becomes farther from the compression (force) applying position. Therefore, those disks located farther from the compression applying position are compressed to a smaller degree than those located closer to the compression applying position.
Obviously, uneven compression of the porous disks results in that the porosity of the roll body varies along the length of the roll, consequently causing uneven liquid absorption or supply. Further, the hardness of the roll body also varies along the length of the roll, so that the roll body comes into uneven rolling contact with the object surface to result also in non-uniform liquid absorption or supply. Such uneven liquid absorption or supply leads to inappropriate surface treatment, or necessitates repetition of the same surface treatment before achieving an acceptable result.